telephony Slide 1

ear

T

R

mouthDTMF

ringer

C

dial switch

SLIC

TelephoneSubscriber

Line

CO(Local Exchange)

cradle switch

off-hookon-hook

tip (+)

ring (-)

Basic TelephoneBasic Telephone

telephony Slide 2

hybrid

TTelephone

currentdetector

~

ring switch

ringgenerator

(100Vrms 25 Hz) -48 VDC

SubscriberLine

crossconnect switch

Subscriber Line Interface Circuit

Processor

control channel

Call Statesidle on hookdialing dialing in progresscalling after dialingringing incoming callcalled call in progress

tip (+)

ring (-)

Subscriber Line InterfaceSubscriber Line Interface

telephony Slide 3

The PSTN circa 1900The PSTN circa 1900

pair of copper wires

“local loop”

manual routing at local exchange office (CO)

• Analog voltage travels over copper wire end-to-end • Voice signal arrives at destination severely attenuated and distorted

• Routing performed manually at exchanges office(s)• Routing is expensive and lengthy operation• Route is maintained for duration of call

PSTN Review

telephony Slide 4

MultiplexingMultiplexing1900: 25% of telephony revenues went to copper mines standard was 18 gauge, long distance even heavier two wires per loop to combat cross-talk needed method to place multiple conversations on a single trunk

1918: “Carrier system” (FDM) 5 conversations on single trunk later extended to 12 (group) still later supergroups, master groups, supermaster groups

1963: T-carrier system (TDM) T1 = 24 conversations per trunk later T3 = 28 T1s still later SDH rates with 1000s of conversations per trunk

PSTN Review

f

channels

t

timeslots

telephony Slide 5

Cross-connect switchCross-connect switch

Complexity increases rapidly with size

1 2 4 5 6 7 83

1

2

3

4

5

6

7

Analog Crossbar switch Digital Cross-connect (DXC)

processor

t1 2 3 4 5

t2 1 5 4 3

telephony Slide 6

SLIC

PCMMUX

Hybrid

SLIC

TDMport

TDMport

TDMport

TDMport

Tone&

Ann

DTMFdecode

Processor

SubscriberLines

Hybrid

2w 4w

DXC

E1/T1E1/T1

Basic TDM COBasic TDM CO

telephony Slide 7

PSTN TopologyPSTN Topology

Many local telephone exchanges had sprung up

Bell Telephone acquired them

and interconnected them for long distance

LocalExchange Local

Exchange

LocalExchange

local loop

Long distancenetwork

trunkcircuit

subscriber line

PSTN Review

telephony Slide 8

Old US PSTNOld US PSTN

Class 3 Class 3

Class 2 Class 2

Class 1

Class 3

Regional centers

Sectional centers

Primary centers

Toll (tandem) offices

Central (end) offices

Class 4

last mile subscriber lines local loop

circuits,trunks

Class 5 switch is the sole interface to the subscriber lines

PSTN Review

Class 5 Class 5 Class 5 Class 5 Class 5

Class 4 Class 4 Class 4

telephony Slide 9

Numbering PlansNumbering Plans

An E.164 International Number has the format :

Country-Code Area-Code Exchange-Code Line-Number

EX: 972 2 588 9159

Country-Codes 1 : N America (US, CA, Caribbean) 1 digit 2 : Africa 2 or 3 digits 20 Egypt 27 South Africa 235 Chad 3 : Europe 2 or 3 digits 31 Netherlands 354 Iceland 4 : Europe 2 or 3 digits 44 UK 49 Germany 420 Czech Republic 5 : S America 2 or 3 digits 54 Argentina 595 Paraguay 6 : Australia & S Pacific 2 or 3 digits 61 Australia 675 Papua 7 : Russia 1 digit 8 : China & N Pacific 2 or 3 digits 86 PR China 855 Cambodia 9 : Middle East 2 or 3 digits 90 Turkey 972 Israel

MAXIMUM 15 DIGITS

telephony Slide 10

SignalingSignaling

PSTN with automatic switching requires signaling

The present PSTN has thousands of features

and all require signaling support

Examples:

On-hook / off-hookPulse / Tone dialingReceiver off-hookCall waitingCaller number identificationCall forwardingHook-flash

Fax transmission detectInter-CO messagingEcho cancellationVoice mailConference callsCoin-dropBilling

PSTN Review

telephony Slide 11

Signaling MethodsSignaling Methods

Signaling can be performed in many ways

Analog voltage signaling loop-start, ground-start, E&M

In-band signaling DTMF, MFR1, MFR2

Channel associated signaling (CAS) AB bits, ABCD bits

Common channel signaling (CCS) SS7, QSIG

– Trunk Associated CCS

– Separate signaling network CCS

PSTN Review

telephony Slide 12

Subscriber - Exchange Subscriber - Exchange Signaling*Signaling*

On/off hook DC current flow

Dial-tone 350+440 continuous

Pulse Dialing DTMF L1 697 L2 770 L3 852 L4 941 H1 1209 H2 1336 H3 1477 H4 1633

Ring AC with cadence

Ring-back 440+480 with 2:4 cadence

Busy 480+620 with 1/2:1/2 cadence

Trunk busy 480+620 but 0.2:0.3 cadence

Receiver off-hook 1400+2060+2450+2600 with ).1:0.1 cadence

Invalid (nonworking, unobtainable) number announcement, SIT-tone

* US cadences - national differences

H1 H2 H3 H4

L1 1 2 3 A

L2 4 5 6 B

L3 7 8 9 C

L4 * 0 # D

make

break

>700ms>100ms (60:40)off-hook

telephony Slide 13

Subscriber - SubscriberSubscriber - Subscriber Subscriber - Exchange Subscriber - Exchange

SignalingSignaling Hook-flash on-hook for 0.1 - 1 sec

Echo suppressor disabler 2100 continuous

– FAX CED 2100 2.6 - 4 sec

– Modem ANS 2100 with phase reversals every 450 ms for 3.3 s

– ANSAM 2100 with phase reversals and 15 Hz AM modulation

CNG – FAX 1100 with 0.5:3 cadence

– Modem 1300 with 0.5:2 cadence

Caller Line Identification (CLI/CND) – 1200 bps FSK (V.23) data signal 1300 = 1 2100 = 0

– Between 1st and 2nd ringsRING RINGCLI>300 ms > 475 ms

telephony Slide 14

Supervision proceduresSupervision procedures

FXO/FXS Foreign Exchange Office / Foreign Exchange Subscriber

FXS is like exchange - provides voltage, ringing, dialtone FXO is like a phone - requires voltage, detect ring, etc. When connecting PBX to CO, PBX is FXO, CO is FXS

Analog loop start Digital loop start Analog ground start Digital ground start E&M (wink)

telephony Slide 15

Interexchange SignalingInterexchange Signaling

CAS - R2

CAS - R1

CCS - SS7

telephony Slide 16

Optimized Telephony RoutingOptimized Telephony Routing

Circuit switching (route is maintained for duration of call)

Route “set-up” is an expensive operation, just as it was for manual switching

Today, complex least cost routing algorithms are used

Call duration consists of set-up, voice and tear-down phases

PSTN Review

telephony Slide 17

The PSTN circa 1960The PSTN circa 1960

local loop

subscriber line

automatic routing through universal telephone network

• Analog voltages used throughout, but extensive Frequency Division Multiplexing • Voice signal arrives at destination after amplification and filtering to 4 KHz

• Automatic routing• Universal dial-tone• Voltage and tone signaling• Circuit switching (route is maintained for duration of call)

trunks

circuits

PSTN Review

telephony Slide 18

The Digitalization of the PSTNThe Digitalization of the PSTN

Shannon (Bell Labs) proved

is better than

and the PSTN became digital

Better means More efficient use of resources (e.g. more channels on trunks)

Higher voice quality (less noise, less distortion)

Added features

Digital

Communications

Analog

Communications

PSTN Review

telephony Slide 19

TimingTiming

In addition to voice, the digital PSTN transports timing

This timing information is essential because of

– the universal use of TDM

– the requirement of accurate playback (especially for fax/modem)

Receiving switches can recover the clock of the transmitting switch

Every telephony network has an accurate clock called “stratum 1”

Clocks synchronized to it are called “stratum 2”

Clocks synchronized to them are called “stratum 3”

and so on

PSTN Review

telephony Slide 20

The Present PSTNThe Present PSTN

subscriber line

• Analog voltages and copper wire used only in “last mile”, but core designed to mimic original situation• Voice signal filtered to 4 KHz at input to digital network

• Time Division Multiplexing of digital signals in the network• Extensive use of fiber optic and wireless physical links• T1/E1, PDH and SONET/SDH “synchronous” protocols

• Signaling can be channel/trunk associated or via separate network (SS7)

• Automatic routing• Circuit switching (route is maintained for duration of call)• Complex routing optimization algorithms (LP, Karmarkar, etc)

PSTN Network

core

backbone

PSTN Review

telephony Slide 21

Nonvoice servicesNonvoice services

The PSTN can even be used to transport non-voice signals

such as FAX or DATA

These services disguise themselves as voice by using a modem Proper timing is essential Special signaling is required

– turn off LEC

– turn off call waiting

– service recognition

PSTN

VoP course

PSTN Review

– capabilities negotiation

– mutual identification– end of page/document

– modem recognition – modem training– data compression

telephony Slide 22

DDigitaligital L Loop oop CCarrierarrier

Pushes the digital PSTN closer to customer

AT&T SLC-40, SLC-96, Nortel DMS P-phone, “pair-gain”

TR-08 multiplex 96 lines on: Mode 1: 4 T1s Mode 2: 2 T1s (2:1 concentration)

GR303/V5.1/V5.2 multiplex up to 2048 lines

CLASS 5

Street cabinet

UTP/coax/fiber

UTP

pedestal

TR-08 Mode 1 pair-gain:Replace 96 pairs with 5 T1s (one spare for “span protection”) 96 – 10 = 86

TR-08 Mode 2 pair-gain:Replace 96 pairs with 2 T1s (without “span protection”) 96 – 4 = 92

Access Network

CPEFTTB/FTTC

PSTN Review